It is very important that there is a remote capability for real-time backup protection of direct access storage device (DASD) data at a distant location. This is typically referred to as remote duplexing or remote data copying. It is important to have this type of service to provide disaster backup to data. Many entities would want backup service to protect them in the case of disaster such as a fire or the like. Most customers want this backup capability to be DASD storage based and application independent. That is, they do not want to have a separate backup process per application or per data base system.
Most important is that a customer requires that the data copy at the remote location be consistent, that is, that the data can be used for recovery. Since means for consistent data copy must be accomplished in view of there being multiple DASD subsystems connected to multiple host systems, a design for remote data duplexing requires a means for ensuring system-wide sequence registration of write data in which DASD control units sends an explicit token or information packet to a sequencer program for each write operation that is executed. To ensure accurate sequencing, that registration process must be synchronous with the write operation being registered, that is, the write operation cannot be completed by the control unit until the information packet has been accepted by the sequencer.
Accordingly, for an information packet synchronous with the host write operation, the sending of the information packet by the control unit must be performed on a different internal storage path, not the storage path that is processing the write operation. In an ideal situation, the time to acquire a storage path, connect to a channel to the global sequencer and send the information packet would be completely overlapped by the data transfer of the write data. Any delay in acquiring a storage path or in connecting to the channel may cause the information packet send time to be greater than the write data transfer time thus delaying the completion of the primary write operation.
Although the data duplexing can be asynchronous, there is a synchronous component associated with each information packet. This synchronous component affects the efficiency of the overall system. This problem can be particularly acute during those periods of high update activity because it is very difficult for multiple events to be communicated to a sequencer in a single communication. Hence, what is needed is a system for improving the efficiency of an asynchronous remote data duplexing system. The system should be one that does not compromise the integrity of the data being copied and also does not affect the performance of the system.